Conventional lithographic printing plates, such as those typically used by both newspaper and commercial printers, are usually made of grained, anodized aluminum substrate which has been coated with a light sensitive coating. The grained, anodized aluminum is generally post treated to enhance hydrophilicity of the substrate sheet prior to the application of the light sensitive coating. Solutions which are useful for post treatment include, for example, sodium silicate and polyvinylphosphonic acid.
Graining of aluminum is accomplished in a variety of ways, including rotary brush graining, chemical graining and electrochemical graining. It is possible to use more than one of these techniques in the production of lithographic substrate. A grained surface has better adhesion to light sensitive coatings and carries fountain solution in the background areas of the plate on the press more efficiently than the ungrained surface.
Anodizing is the process of electrolytically generating aluminum oxide on the surface of the aluminum sheet. Commonly used anodizing electrolytes include sulfuric acid and phosphoric acid. Since anodic aluminum oxide is harder and more abrasion resistant than aluminum, an anodized printing plate has a greater press life than a bare plate.
Computer-to-plate systems using infrared lasers are now available for imaging printing plates. By imaging directly on the plate, the use of photographic negatives is eliminated.
U.S. Pat. No. 4,731,317 to Fromson et. al. discloses a printing plate based on a substrate which is brush grained in a slurry comprising alumina, followed by successive treatments in dilute sodium hydroxide and nitric acid, and subsequent anodizing to achieve an oxide coating weight of 1.5 milligrams per square inch. The substrate may also be silicated after anodizing to improve hydrophilicity in accordance with U.S. Pat. No. 3,181,461. The anodized plate is coated with a diazo resin which is transparent to the radiation of a YAG infrared laser (1064 nanometers), but is sensitive to the longer wavelengths generated within the areas of the anodic oxide exposed to the laser. The theory is that the grained surface traps the laser radiation and re-emits the energy as longer wavelengths. This light trapping property must be enhanced by the addition of carbon black to the diazo. The diazo is rendered insoluble where the plate is exposed to the laser. Following laser exposure, the unexposed diazo is removed with a solvent to reveal hydrophilic oxide in the background. Because the non-imaged areas are removed with a solvent, the plate is described as negative working.
U.S. Pat. No. 4,731,317 mentions that the diazo may partially ablate when the level of the laser radiation is relatively high. Such ablation is undesirable since the areas exposed to the laser radiation are to remain on the plate as the ink bearing image after processing in the developing solution.